Amethyst
| Amethyst | |
|---|---|
Amethyst cluster from Artigas, Uruguay | |
| General | |
| Category | Tectosilicates, quartz variety |
| Formula | Silica (silicon dioxide, SiO2) |
| Crystal system | Trigonal |
| Crystal class | Trapezohedral (32) |
| Space group | P3221 (no. 154) |
| Identification | |
| Color | Purple, violet, dark purple |
| Crystal habit | 6 sided prism ending in 6 sided pyramid (typical) |
| Twinning | Dauphine law, Brazil law, and Japan law |
| Cleavage | None |
| Fracture | Conchoidal |
| Mohs scale hardness | 7 (lower in impure varieties) |
| Luster | Vitreous/glassy |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.65 constant; variable in impure varieties |
| Optical properties | Uniaxial (+) |
| Refractive index | nω = 1.543–1.553 nε = 1.552–1.554 |
| Birefringence | +0.009 (B-G interval) |
| Pleochroism | Weak to moderate purple/reddish purple |
| Melting point | 1650±75 °C |
| Solubility | Insoluble in common solvents |
| Other characteristics | Piezoelectric |
Amethyst is a violet variety of quartz. The name comes from the Koine Greek αμέθυστος amethystos from α- a-, "not" and μεθύσκω (Ancient Greek) methysko / μεθώ metho (Modern Greek), "intoxicate", a reference to the belief that the stone protected its owner from drunkenness. Ancient Greeks wore amethyst and carved drinking vessels from it in the belief that it would prevent intoxication.
Amethyst, a semiprecious stone, is often used in jewelry.